1. Field of the Invention
The present invention relates to a master-slave-type control system having a back-up function, and particularly to a master-slave-type control system for effecting back-up switching from a normal or running supervisory control station to a stand-by supervisory control station without using exclusive signal lines.
2. Description of the Prior Art
There is known a master-slave-type control system in which there is a normal supervisory control station as a master device, one or more control terminal devices as slave devices, and a stand-by supervisory control station for effecting operation of the normal supervisory control station in place thereof are connected with a common transmission passage. In this system, back-up control for detecting an abnormality of the normal supervisory control station and switching of control by the normal supervisory control station to that by the stand-by supervisory control station is carried out in the following manner.
Namely, an exclusive information collection device for collecting tracking information provided on the transmission passage collects and stores control information always transmitted from a control terminal device to the normal supervisory control station. A watchdog timer is provided in the normal supervisory control station. The watchdog timer detects an abnormality of the normal supervisory control station, transmits its output in the form of a contact output, and transmits the output to the stand-by supervisory control station through signal lines. Then, the abnormal station is separated based on the abnormal output thereof, and at the same time, the tracking information is downloaded from the exclusive collection device to the stand-by supervisory control station so as to make the stand-by station effect the function of the normal station in place thereof.
However, in the master-slave-type control system, when the stand-by station executes as a proxy for any running station of a plurality of the normal supervisory control stations, an abnormal detection process circuit for a plurality of the supervisory control stations, that is, the watchdog timer and collection circuit, becomes complex as the number of the supervisory control stations increases. With said complexity, the exclusive signal line would require complex wiring to the abnormal detection process circuit and the respective supervisory control stations.